Prior art package structures for CCD and CMOS image sensors rely, for protecting the sensor portion from foreign matter such as moisture and dust, on the method of bonding the structure to protective glass with a photo-curable photosensitive adhesive composition in liquid or film form which is applied by printing techniques, typically dispensing.
The formation of the adhesive layer by the photolithography process starts with the step of coating the photosensitive adhesive composition or disposing a photosensitive adhesive film on a silicon substrate or glass substrate to form a photosensitive adhesive layer. Next, the photosensitive adhesive layer is exposed, baked and developed before a protective glass substrate (silicon substrate) is placed in close contact with the adhesive layer. Heat and pressure are applied to establish a bond between the adhesive layer and the protective glass. This is followed by heat curing and dicing, obtaining a hollow structure package. As the photosensitive resin composition, there are known a photosensitive resin composition comprising an acrylic resin, a photo-polymerizable compound and a photo-polymerization initiator (Patent Document 1: JP-A 2002-351070), a photosensitive resin composition comprising a photosensitive modified epoxy resin, a photo-polymerization initiator, a diluent solvent, and a thermosetting compound (Patent Document 2: JP-A 2003-177528), and the like.
Also, an adhesive dry film comprising a polyimide resin, a curable compound and a silane coupling agent is disclosed as a mere thermosetting adhesive (Patent Document 3: JP-A 2003-253220).
However, the photosensitive resin compositions of Patent Documents 1 and 2 are insufficient in properties needed as adhesive including adhesion, hermetic seal after heat curing, and low hygroscopicity and have the drawbacks of high cost and low productivity because three steps of exposure, bake and development are essential for the manufacturing process. Also the thermosetting adhesive of Patent Document 3 is insufficient in such properties as adhesion, heat resistance and light resistance.
Further, the 3D packaging technology using through-silicon via (TSV) recently becomes the mainstream of CMOS image sensor manufacture. In the 3D package, the silicon substrate is bonded to protective glass via heat curing, after which the silicon substrate must be ground on the back surface until the silicon substrate is thinned to a thickness of 100 μm or less. At this point, it is important that the thin wafer can be ground without fissure, and a new problem arises that the bonded substrate is largely warped. The problem becomes outstanding particularly with a large diameter wafer of 8 inches or more.
An epoxy-containing thermosetting silicone polymer is disclosed in Patent Document 4: JP-A 2012-188650 as effective for suppressing the warpage of bonded substrate. Although the substrate bonded using this silicone composition is reduced in warpage, a problem arises that its spectroscopic properties are substantially degraded in a high-temperature/long-term heat resistance test (e.g., more than 50 hours). Similar silicone polymers are known from Patent Document 5: JP-A 2012-229333 and Patent Document 6: JP-A 2013-082801. They exhibit good bonding, heat resistance and back grinding, but fail to fully meet necessary properties as a permanent film because of thermoplastic nature.